Chlorine dioxide, useful for bleaching wood pulps, is known to be produced by a number of processes, many of which are practised on an industrial scale, on site at pulp mills. The basis of most of these processes is reduction of sodium chlorate in an acid aqueous reaction medium.
Most commonly in industrial applications, the acid is provided by sulphuric acid and the reducing agent is chloride ions. The reaction whereby chlorine dioxide is fromed is depicted by the following equation: EQU ClO.sub.3.sup.- +Cl.sup.- +2H.sup.+ .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O
The chloride ion reductant may be added to the reaction medium from external sources, as in the commercial processes known as the "R2" and "R3" processes, or may be produced in situ by reduction of the chlorine, as in the commercial processes known as the "Mathieson" and "Solvay" processes, wherein sulphur dioxide and methanol respectively are used to provide the chlorine reduction. These processes are based on the continuous flow of reactants to the generator and result in the production of sodium sulphate as a by-product.
In the R3-process, water is evaporated from the reaction medium while a subatmospheric pressure is applied to the reaction medium, enabling low acid normalities to be used and the precipitation of anhydrous neutral sodium sulphate from the reaction medium to be effected. In the other commercial processes mentioned above, however, namely the R2-process, Mathieson-process and Solvay-process, high total acid normalities are used and spent highly acidic reaction medium containing unreacted sulphuric acid and by-product sodium sulphate is allowed to overflow or otherwise exit the reactor.
One common procedure for processing the latter aqueous acid effluent stream which has been adopted is to feed the aqueous effluent to the recovery system of the pulp mill as a sulphur make-up chemical. However, this procedure can lead to excess sulphidity, which upsets the chemical balance of the pulp mill and hence is undesirable. Some other acid effluent utilizations, including cascading to a crystallizing R3-type chlorine dioxide generator (as suggested in U.S. Pat. No. 3,446,584) and utilization in tall oil separation, are possible, but are not applicable in many mill conditions and additional capital equipment is required.
There exists a need, therefore, for a process for treating the sulphuric acid- and sodium sulphate-containing effluent stream from a non-evaporating chlorine dioxide generator to effect recovery of sulphuric acid therefrom for utilization in any desirable manner, for example, by recycling to the chlorine dioxide generator.
One prior suggestion for the treatment of such effluent stream is contained in Lobley et al U.S. Pat. No. 4,104,365, wherein it is suggested to treat the sulphuric acid- and sodium sulphate-containing effluent stream with water and methanol to precipitate sodium sulphate therefrom. Following separation of the precipitated sodium sulphate, the methanol is stripped from the liquid phase for reuse and the sulphuric acid is concentrated to the desired concentration.
This prior art procedure suffers from a number of drawbacks which have severely limited its commercial implementation. The procedure requires physical separation of the precipitated sodium sulphate, which often is difficult to perform effectively, especially when sodium dichromate is present, which is the case when cell liquor is used as the sodium chlorate feed to the chlorine dioxide generator. In addition, the procedure requires a stripper to enable the methanol to be removed from the liquid phase and special handling equipment for the collecting and condensing the highly volatile and inflammable methanol vapors.